Born in Rome in the region of the Via Lata, Valentine was the son of a Roman noble called Leontius. Showing an early aptitude for learning, he was moved from the school attached to the Lateran Palace and, according to the Liber Pontificalis, was made a Deacon by Pope Paschal I (817-824). His biographer in the Liber pontificalis praises his piety and purity of morals, which won him the favor of Paschal I, who raised him to the rank of Archdeacon. He also was clearly favoured by Paschal’s successor, Pope Eugenius II, to the point where rumours were circulated that Valentine was really the son of Eugenius. According to Louis-Marie DeCormenin, other rumours declared that Valentine and Eugenius were involved in an illicit relationship.

With the death of Eugenius, the Roman clergy, nobility and people all acclaimed Valentine as being the most worthy to occupy the Apostolic See. They took him from the Basilica di Santa Maria Maggiore, and installed him in the Lateran Palace, ignoring his protests. In their haste, they enthroned him before he was consecrated a priest; this was an unusual reversal of the normal proceedings, and in fact was the first time it had happened in the recorded history of the papacy, although it would be repeated during the pontificate of Pope Benedict III. On the following Sunday, he was formally consecrated bishop at St. Peter's Basilica. There were no imperial representatives present during the election, and Valentine had no opportunity to ratify his election with the emperor, as he was dead within five weeks, dying on 10 October 827.